In Phase Shift Keying (PSK), orthogonal BPSK (Binary PSK) waves can be synthesized to obtain a QPSK (Quadriphase PSK) wave having excellent noise margin and a large capacity of transmitted information. Also, in Amplitude Shift Keying (ASK), the orthogonal components can be synthesized to obtain a QAM (Quadrature Amplitude Modulation) wave, in particular, a 16 QAM wave which is currently widely used.
A general arrangement of such a quadrature modulation circuit is illustrated in FIG. 1. The carrier frequency wave generated by an oscillator 1 is input to a mixer 2 which mixes the carrier frequency wave and a baseband signal P1. The carrier frequency wave is also input through a 90.degree. phase shifter 4 to a mixer 3 which mixes a baseband signal P2 and the carrier frequency wave whose phase has been shifted by 90.degree.. The respective mixed waves output from the mixers 2 and 3 are added in an adder 5 to be fed out to a power amplifier (not shown) as a modulated carrier wave (e.g. a QPSK wave).
In this conventional quadrature modulation circuit, the carrier frequency wave from the oscillator 1 is modulated according to the baseband waves P1 and P2 to obtain a modulated carrier wave for transmission. This modulated carrier wave, however, is shifted in frequency over a certain bandwidth, and further this frequency shift is very close to the carrier frequency. Therefore, if the modulated carrier wave for transmission is amplified in power, then part of the amplified carrier wave is leaked back to the oscillator 1 to deteriorate the stability of the transmission carrier wave. In particular, in the case of a portable radio transmitter, since it is difficult to isolate the oscillator which generates the carrier frequency wave from the power amplifier which amplifies the transmission wave, the stability of the transmission wave is remarkably degraded.
It is an object of the present invention to provide a quadrature modulation circuit which allows the stability of the transmission wave to be improved with a simple structure.